Friday, November 2, 2007

It has been a long journey to discover neutrinos, not only from the experimental point of view but also from the theoretical standpoint. All of it started when Pauli proposed the existence of a new charge-neutral, massless, weakly interacting new particle to explain energy conservation in beta decay (radioactive process). You can immediately see the trouble with that particle: it is undetectable... almost. People started thinking about possible ways to measure the neutrino and sources of them. It turns out the sun makes millions and millions of them so they now look at those neutrinos.

The problem in question is not a simple one and the experiment requires a lot of engineering work. The "detector" consist of a large volume of heavy water, though water can also be used. How much water? About 1000 tons of it. But this is not the end of the story, to make matters worse, it needs to be underground ~1.5-2 miles below the ground to shield it from the cosmic radiation which also makes a count in the detector. Then, it has to be all clean, pure water, no dirt or dust, etc. which sounds easy but underground is kind of not clean right? Now, it comes the detection part: they detect something called Cherenkov radiation (produced when a charged particle, say an electron, travels faster than light inside an insulating medium) using photomultipliers, something like 9500 at SNO. The problem here is that many things make a photomultiplier detect a count, I think at SNO they got something like half a billion counts but only about 3000 were possibly neutrinos, which means that once you have "data" you have to sort it out into probabilities of it being a neutrino. This task takes more than a year of work.

They initial prediction include three neutrinos, one for each lepton (electron, muon, tau). We now know that they do come in three (named 1,2,3), but not directly the ones initially thought of. There are three quantum-mechanical wave functions that superpose (they combine) to form something like the classical "beats" with sound waves. The superposition of those give rise to the other three. Also, they do have a mass but a minuscule one. This particular property forced physicist to reformulate the most of the neutrino theory, since the massless condition implies certain theoretical results, namely that neutrinos cannot change flavor.

There are many experiments taking place or being planned to extend the research in neutrino physics, specially for those coming directly from the sun. If you are interested check out the wikipedia entry, it has links to the different underground labs and to the professors and universities where they work. Some of them have explicit adds for grad and/or undergrad students.